I just used a butter knife wrapped in a Clorox wipe... being careful not to have the metal show so that it will scrape. when the wipe comes out black, rotate the wipe so that it will be able to clean. I used 5 wipes rotating. I could not figure out how to open the thing either so this was my solution.

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Is this purifier wet at all, or been in any heavy areas of moisture. This purifier is very undependable with moisture. It may just have to dry out. Let me know if this solves your problem or we can explore further options.

Unplug the unit before cleaning. Wipe off the cabinet with a soft cloth and a mild non-abrasive cleanser.

To clean the electronic collector, loosen the front grill retainer screw then press in the lock and remove the grill. Pull out the collector by the tabs on the sides using both hands. Snap off the charcoal filter from the back of the collector. Then slide off the pre-filter. Spray the pre-filter and the collector with the Oreck Assail-a-Cell cleanser and then let stand for a few minutes before rinsing with water. Otherwise, soak the pre-filter and the collector in warm water (bathtub or sink) with mild dish detergent. Soak for 2-3 hours. Rinse well.

Then allow the parts to dry for completely before reassembling the air purifier. Air dry only.

Here's how you can clean the machine, and I have copied the link to the owners manual below for your convenience.

Cleaning
The Cabinet
You can clean the air purifier cabinet with mild, non-abrasive cleanser and a
soft cloth. IMPORTANT: Unplug the unit from its electrical power source.
Do not spray cleansers or furniture polish into the louver or grill areas.

Collector Cell
Indicator light is green under normal operation and flashes red
when the cell requires cleaning. To clean, proceed as follows:
The red "WASH CELL" light will flash or illuminate when it is
time to clean your cell. However, it is recommended that every four to
eight weeks or if snapping sounds become frequent, clean your
electronic air purifier. Your electronic air purifier is still filtering the air
when the red light illuminates. Cleaning your electronic air purifier is a
snap too... and a must for efficient operation.
1. Turn the unit off and unplug it from the electric power source. If wall
mounted, remove unit from wall.
2. Remove front grill by unscrewing the retainer screw under the grill on
the cabinet bottom. (fig. 1)
3. Pull the cell out of the cabinet by using the two small handles on the
front of the cell. (fig. 2)
4. Snap off the charcoal odor absorber from the back of the cell. Change
as necessary for continued odor removal. (fig. 3)
5. Slide off pre-filter. This can be cleaned with the cell and reused. (fig. 4)
NOTE: Handle the electronic cell with care. Damaged metal plates
and/or broken wires will cause the air purifier to malfunction. (Do
not wash charcoal odor absorber; it is disposable.)
Cleaning with Assail-A-Cell® Cell Cleaner:
6. Using Assail-A-Cell® Cleaner, thoroughly saturate the cell and prefilter.
Be sure to spray between each cell plate and cover every
surface. Allow product to soak for a few minutes, not allowing it to dry.
Rinse thoroughly with water. For extremely heavy deposits, repeat the
process.
IMPORTANT: Dry the collection cell thoroughly outside of the
air purifier before reinstalling it into the air purifier. Ensure
that all areas including corners and areas that may trap
water are dry before reinstalling into the air purifier. DO NOT
place a wet or damp collection cell in the air purifier
4. Snap off the charcoal odor absorber from the back of the cell. Change
as necessary for continued odor removal. (fig. 3)
5. Slide off pre-filter. This can be cleaned with the cell and reused. (fig. 4)
NOTE: Handle the electronic cell with care. Damaged metal plates
and/or broken wires will cause the air purifier to malfunction. (Do
not wash charcoal odor absorber; it is disposable.)
Cleaning with Assail-A-Cell® Cell Cleaner:
6. Using Assail-A-Cell® Cleaner, thoroughly saturate the cell and prefilter.
Be sure to spray between each cell plate and cover every
surface. Allow product to soak for a few minutes, not allowing it to dry.
Rinse thoroughly with water. For extremely heavy deposits, repeat the
process.
IMPORTANT: Dry the collection cell thoroughly outside of the
air purifier before reinstalling it into the air purifier. Ensure
that all areas including corners and areas that may trap
water are dry before reinstalling into the air purifier. DO NOT
place a wet or damp collection cell in the air purifier.